drm/panfrost: Simplify the reset serialization logic

Now that we can pass our own workqueue to drm_sched_init(), we can use
an ordered workqueue on for both the scheduler timeout tdr and our own
reset work (which we use when the reset is not caused by a fault/timeout
on a specific job, like when we have AS_ACTIVE bit stuck). This
guarantees that the timeout handlers and reset handler can't run
concurrently which drastically simplifies the locking.

v5:
* Don't call cancel_delayed_timeout() in the reset path (those works
  are canceled in drm_sched_stop())

v4:
* Actually pass the reset workqueue to drm_sched_init()
* Don't call cancel_work_sync() in panfrost_reset(). It will deadlock
  since it might be called from the reset work, which is executing and
  cancel_work_sync() will wait for the handler to return. Checking the
  reset pending status should avoid spurious resets

v3:
* New patch

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-10-boris.brezillon@collabora.com

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